The applicant of the present invention has proposed a molten iron manufacturing process, as a new iron making process that takes the place of a known blast-furnace process or smelting reduction process, achieved by combining a rotary hearth furnace in which solid reduced iron is produced by thermally reducing carbon composite iron oxide agglomerates composed of an iron oxide source and a carbonaceous reductant, with an iron bath furnace in which the solid reduced iron is melted using, as a heating source, the heat generated when a carbonaceous material is burned with top blowing oxygen gas (refer to Patent Documents 1 and 2). However, since a large amount of oxygen gas is consumed in the iron bath furnace in this process, molten iron cannot be manufactured stably if oxygen gas is not sufficiently supplied. Therefore, instead of the iron bath furnace, the development of a melting furnace that uses a heating source other than combustion heat has been required.
In the molten iron manufacturing process, since the solid reduced iron includes gangue contained in the iron oxide source and ash of the carbonaceous reductant, a large amount of molten slag (hereinafter may be simply referred to as “slag”) is produced when the solid reduced iron is melted in a melting furnace. In particular, when, for example, electric furnace dust containing a large amount of slag component is used as the iron oxide source, 700 kg or more of slag or sometimes 1000 kg of slag per ton of molten iron is produced. Once such a large amount of slag is abnormally formed in the furnace, it is too late to remedy the situation. The slag forming causes the interruption of the operation and the slag brought into an exhaust gas system blocks the exhaust gas system. In addition, when slag is tapped, the slag is cooled and solidified because the slag whose weight is decreased due to its forming has small thermal capacity, whereby a tap hole may be blocked.
On the other hand, an example of a method for manufacturing molten steel by smelting solid reduced iron includes a method in which a large amount of solid reduced iron is blended with scraps and then smelted in a tilting electric arc furnace, and tapping of molten steel and tapping of molten slag are performed by tilting the furnace. The solid reduced iron used in this method is manufactured by reducing high-grade iron ore pellet and lump ore using a reducing gas obtained by modifying natural gas. The amount of molten slag produced in this method is about 100 to 150 kg per ton of molten steel, and the amount is much smaller than that produced in the molten iron manufacturing process. In this method, therefore, troubles caused by the slag forming and the solidification of slag during tapping of slag rarely occur and the slag forming and the solidification of slag are not problematic.
Thus, it is considered that molten iron is manufactured by this method with the solid reduced iron used in the molten iron manufacturing process, instead of the solid reduced iron used in this method. In this case, however, a large amount of molten slag is produced at the melting stage of the solid reduced iron, which increases the possibility of troubles caused by slag forming. In addition, since tapping of molten iron and slag needs to be performed by tilting a furnace in every charge, the molten iron flows out during tapping and heat radiation loss increases during tapping, which poses a problem in that the productivity of molten iron is reduced.
Another example of the method for manufacturing molten steel by smelting solid reduced iron includes a method in which molten steel is manufactured by smelting solid reduced iron in a submerged-arc furnace with a fixed furnace body using resistance heating from an electrode immersed in a slag layer, and molten steel and molten slag are intermittently discharged from a tap hole and a cinder notch disposed on the furnace side (e.g., refer to Patent Document 3).
In this method, tilting of a furnace is not necessary during tapping of molten steel and molten slag. Therefore, smelting can be continued even during tapping, but the productivity of this method is low because resistance heating is used. If the size of the furnace is increased to compensate for the low productivity, operating cost is increased due to an increase in power consumption and the cost of equipment is also increased.
An example of a method for preventing the blocking of a tap hole and a cinder notch caused by cooling of molten iron or slag includes a method in which the temperatures of a furnace bottom, a tap hole, and a cinder notch are increased by winding an induction coil on the bottom of a blast furnace or a blast furnace-type melting furnace to generate heat (refer to Patent Document 4).
However, in this method, an expensive induction heating apparatus is required to increase the temperature of slag, which not only increases the cost of equipment but also requires efforts for maintenance.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-176170    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-257545    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 50-134912    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2001-241859